Waves in Shallow Water & Tsunamis

Question 1

The longer the wavelength, the _____ (faster/ slower) the wave travels

Answer 1

faster

Question 2

There is negligible water movement below depth=___ wavelength

Answer 2

1/2

Question 3

When waves enter water shallower than their wave base, they interact with the _______

Answer 3

seafloor

Question 4

What happens when waves interact with the seafloor?

Answer 4

• the seafloor prevents water particles from moving in circular orbits –> particles immediately above the seafloor can only move back and forth
• orbits of molecules become stretched out ovals
• this compression, plus friction, slows the forward motion of the wave

Question 5

For deep-water waves, C is a function of:

For shallow-water waves, C is a function of:

For transitional waves, C is a function of:

Answer 5

wave length

water depth

both water depth and wave length

Question 6

As water depth decreases, wave speed ___

Answer 6

decreases also

for shallow water waves, wave speed is a function of water depth!

Question 7

T/F
All waves in shallow water travel at the same speed in the same depth water, regardless of period

Answer 7

true!

Question 8

As waves enter shallower water and are slowed, does their period change? Does the wavelength change?

Answer 8

Period- no

Wavelength- yes. it decreases

Question 9

Do each of these increase or decrease when a wave enters shallow water?
- speed (C)
- wavelength (L)
- height (H)
- steepness (H/L)

Answer 9

• speed (C) decreases
• wavelength (L) decreases
• height (H) increases
• steepness (H/L) increases

Question 10

Explain why wave height and steepness increase in shallow waters

Answer 10

energy must be conserved; kinetic energy is converted to potential energy so height and steepness increase

Question 11

Explain wave refraction

Answer 11

waves usually approach the shore at an angle= one part of the wave is in shallow water & the rest is in deeper water, refracting (bending) the wave

–> as this process continues to bend the wave, the crests tend to become aligned parallel to the shore

Question 12

Wave refraction=

result=

Answer 12

= the bending of shallow water waves due to changes in water depth

result= waves normally end up reaching the shore ~parallel to it

Question 13

surf zone=

Answer 13

the area offshore within which waves are breaking

–> wave steepness increases until the wave becomes unstable and breaks in this surf zone

Question 14

Waves break in different ways, depending on 3 factors that determine how quickly the wave becomes over-steepened and unstable:
1
2
3

Answer 14

wave period
height
slope of the ocean floor

Question 15

Spilling wave break=

Answer 15

seafloor almost flat; crest tumbles down the forward face of the wave (foam); break progressively

Question 16

plunging wave break=

Answer 16

seafloor moderately steep, bottom part of the wave slowed more quickly than the upper part; the crest curls over in front of the wave and plunges downward until it crashes into the trough preceding the wave

Question 17

collapsing wave break=

Answer 17

seafloor steep; lower part of the wave slowed so rapidly that the leading face collapses before the crest arrives (foam); crest collapses behind with little splash

Question 18

surging wave break=

Answer 18

very steep shores; waves appear not to break; simply surge up and down with little bubble production (no foam); most of the wave energy is reflected

Question 19

Why are wave conditions for surfing generally better on the west coast than the east coast of north america?

Answer 19

1. fetch: the pacific is bigger than the Atlantic, so bigger waves can develop
2. beach slopes are generally steeper on pacific coasts, creating plunging breakers
3. onshore winds: winds generally onshore (westerlies), enhancing waves on west coast and weakening them on the east coast

Question 20

If you’re stuck in a rip current, what should you do?

Answer 20

swim parallel to the beach (don’t try to swim against it)

Question 21

Explain how rip currents occur

Answer 21

• the water dumped nearshore by breakers must return offshore
• when waves approach at an angle, water is transported along the beach in the direction of the waves
• eventually water encounters an area where is can flow back out more easily
• wave heights are smaller there
• large amounts of water through a narrow corridor –> fast rip currents

Question 22

_____ are caused by abrupt displacements of ocean water.

Answer 22

Tsunamis

Question 23

Tsunamis originate from sudden changes in the topography of the ____. Give some examples of what could cause these changes

Answer 23

seafloor

• earthquakes (slippage along underwater faults)
• underwater avalanches
• collapse of large oceanic volcanoes
• underwater volcanic eruptions
• meteorite or asteroid impacts (less frequent)

The majority are caused by vertical displacements of seafloor along faults, which change the volume of the ocean basin and affect the entire water column

Question 24

A tsunami behaves as a __-water wave everywhere in the ocean

Answer 24

shallow

Question 25

Tsunamis wave speed is determined by:

They have much ___ (longer/shorter) periods than wind waves

Answer 25

water depth
(because it acts as a shallow water wave)

longer

Question 26

t/f

tsunamis form a huge breaking wave at the shoreline

Answer 26

FALSE

a tsunami doesn’t break at the shoreline –> its wavelength is so long that even a large increase in wave height does not produce steep, unstable waves

Question 27

tsunamis move over __km/hr and have heights of __m or less

Answer 27

700km/hr (212m/s)

wave heights of 1m or less –> in deep water, ships can pass unnoticed

Question 28

Tsunamis are often misnamed ____ ____. Is this accurate?

Answer 28

tidal waves

Not accurate, tsunamis have no relation to tides

They do resemble a sudden, extremely high tide, which is where this name comes from. It’s a strong flood or surge of water that causes the ocean to advance/ retreat dramatically

Question 29

T/F

The first tsunami surge is usually the largest

Answer 29

false

tsunamis are a series of waves –> alternating series of dramatic surges and withdrawals of water (every 5 to 10min)

the first surge is rarely the largest

Question 30

The size of a tsunami partly depends on the size of the _____

Answer 30

earthquake

Question 31

Is the Richter scale linear?

Answer 31

No

An increase of one unit of magnitude (eg 8.1 to 9.1) represents a 10-fold increase in wave amplitude or ~30-fold increase in energy released